Oxidation process in fluidised bed reactor

ABSTRACT

A process for reacting in a fluid bed reactor at least one oxidisable reactant with molecular oxygen in the presence of a catalytically active fluidised bed of solid particles. In the process a molecular oxygen-containing gas having an oxygen concentration greater than that of air is introduced into the fluidised bed whilst the fluidised bed is maintained in a turbulent regime. The process is suitable for oxidation, ammoxidation and carboxylation processes, including the production of maleic anhydride, acrylonitrile, ethylene, acetic acid and vinyl acetate.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 11/541,678,filed Oct. 3, 2006, now abandoned, which is a continuation ofapplication Ser. No. 11/239,137, filed Sep. 30, 2005, now U.S. Pat. No.7,189,871, which is a divisional of application Ser. No. 10/303,769,filed Nov. 26, 2002, now U.S. Pat. No. 7,145,033, the entire content ofwhich is hereby incorporated by reference in this application.

The present invention relates to a process for reacting at least oneoxidisable reactant with molecular oxygen and in particular to reactionin a fluid bed reactor.

BACKGROUND OF THE INVENTION

Reactions of molecular oxygen with oxidisable reactants in the presenceof fluidised beds of catalyst are known. For example, the acetoxylationof ethylene to produce vinyl acetate by reaction of acetic acid,ethylene and an oxygen-containing gas in a fluid bed reactor is knownfrom EP-A-0672453, EP-A-0685449 and EP-A-0847982. The reaction of oxygenwith ethane to produce acetic acid in a fluid bed reactor is known fromEP-A-0546677.

EP-A-0685449 discloses a process for manufacturing vinyl acetate in afluid bed reactor comprising feeding ethylene and acetic acid into thefluid bed reactor through one or more inlets, feeding anoxygen-containing gas into the fluid bed reactor through at least onefurther inlet, co-joining the oxygen-containing gas, ethylene and aceticacid in the fluid bed reactor while in contact with a fluid bed catalystmaterial to enable the ethylene, acetic acid and oxygen to react toproduce vinyl acetate and recovering the vinyl acetate from the fluidbed reactor. According to EP-A-0685449, the oxygen may be added in pureform or as an admixture with inert gas such as nitrogen or carbondioxide. Since the oxygen and hydrocarbons are not mixed until they areboth inside the reactor, catalyst is present when they meet and reactionproceeds immediately, causing the oxygen partial pressure to drop. Thus,an advantage of feeding an oxygen-containing gas to the reactor throughat least one further inlet in addition to the ethylene and acetic acidreactants is that it allows significantly higher levels of oxygen to besafely employed without a high inventory of flammable gas mixtures.

Bubbling bed operation is not appropriate for reactions using molecularoxygen-containing gas because of the possibility of a plume or bubblesof oxygen forming in the bed, which can be unsafe.

There remains a need for an improved fluid bed process in whichmolecular oxygen is reacted with at least one oxidisable reactant.

SUMMARY OF THE INVENTION

It has now been found that if a molecular oxygen-containing gas havingan oxygen concentration greater than that of air is to be fed into afluidised bed comprising catalyst particles in a reactor, the fluidisedbed should be maintained in a turbulent regime.

Thus, according to one aspect of the present invention there is provideda process for reacting in a fluid bed reactor, at least one oxidisablereactant with molecular oxygen in the presence of a catalytically activefluidised bed of solid particles in which process a molecularoxygen-containing gas having an oxygen concentration greater than thatof air is introduced into said fluidised bed whilst said fluidised bedis maintained in a turbulent regime.

Fluidised beds comprising solid particles may be operated in variousregimes—for example, as spouting beds, fast fluidisation (transporting)beds, bubbling beds and turbulent beds etc.

A turbulent regime for a fluidised bed of solid particles is a fluidisedbed in which large discrete bubbles or voids are generally absent andthe bed is substantially uniform. Clusters and strands of particles moveabout violently within the bed making for a general lack of distinctionbetween continuous and discontinuous phases in the bed. In a turbulentfluidised bed, the upper surface is considerably more diffuse than in abubbling bed because of greater freeboard activity.

A turbulent regime has a linear velocity of the fluidising gas (lineargas velocity) which is greater than the transition velocity (U_(k))which is, the gas velocity at which there is a transition from bubblingto turbulent regimes.

Preferably the ratio of transition velocity (U_(k)) terminal velocity(U_(t)) is in the range 0.1:1 to 25:1. Typically, at a pressure in therange 1 to 30 bara, the ratio U_(k):U_(t) may be in the range 0.1:1 to25:1, preferably in the range 0.3:1 to 13:1. For small solid particles(for example, in the range 30 to 150 microns diameter), the turbulentregime has a transition velocity U_(k) which is much greater than theterminal velocity (U_(t)) of the mean particle size of bed material. Aspressure in the fluid bed is increased, the ratio U_(k):U_(t) decreases,for example from 3:1 to 1:1 or from 3.5:1 to 0.5:1.

U_(k):U_(t) is greater for smaller particles than for larger particles.

The use of a turbulent fluid bed provides good mass transfer. Thus, oneadvantage is that large bubbles of molecular oxygen-containing gas arenot formed or present in the bed. Therefore, the potential for gasbubbles forming in the bed having a molecular oxygen concentration in anexplosive region is reduced which has increased safety implications.Also, at the oxygen injection point, there is reduced tendency fordiscrete jet or plume formation because of the greater mixing, so thepotential for adverse consequences in the event of an ignition isreduced or eliminated. Furthermore, the mixing of a turbulent fluid bedcan produce a more homogeneous temperature profile. There is good heattransfer and low tendency for hot spots to form.

The process of the present invention is suitable for the carboxylationof alkenes to produce unsaturated esters by reaction of a carboxylicacid, an olefin and molecular oxygen in the presence of an effectiveamount of an active catalyst, such as for example the reaction of aceticacid, ethylene and oxygen to produce vinyl acetate.

The process of the present invention is also suitable for the reactionin the presence of an effective amount of an active catalyst, ofmolecular oxygen with (a) ethane to produce ethylene and/or acetic acid,(b) with ethylene to produce acetic acid and (c) mixtures of ethane andethylene to produce acetic acid, optionally with ethylene.

The process of the present invention is suitable for the oxidation ofhydrocarbons by reaction with a molecular oxygen in the presence of aneffective amount of an active catalyst, such as for example the reactionof C4's to produce maleic anhydride.

The process of the present invention is suitable for the ammoxidation ofhydrocarbons by reaction with a molecular oxygen and ammonia in thepresence of an effective amount of an active catalyst, such as forexample the ammoxidation of propene or propane to produce acrylonitrile.

Preferably, the fluid bed comprises solid particles with a relativelybroad particle size distribution. Thus, for fluid beds comprising solidparticles having a particle diameter in the range 20 to 300 microns, theparticle diameter size distribution is preferably at least 20 microns,more preferably at least 50 microns and most preferably at least 100microns.

Suitably, the fluid bed comprises solid particles having sizes accordingto one or more of the following independent criteria: (i) at least 65%of the particles have particle diameters in the range 20-120 microns;(ii) less than 15% of the particles have a particle diameter less than45 microns; and (iii) less than 5% of the particles have a particlediameter greater than 105 microns.

The total linear gas flow through the fluidised bed (also referred to asthe superficial velocity) is preferably up to 50 cm/s, more preferablyup to 35 cm/s for the reaction of ethane and/or ethylene with oxygen toproduce acetic acid and/or ethylene and more preferably up to 30 cm/sfor the production of vinyl acetate from ethylene, acetic acid andoxygen.

When the process of the present invention is used for the production ofvinyl acetate by the reaction of ethylene and acetic acid with molecularoxygen in a fluid bed reactor in the presence of a catalytically activefluidised bed of solid particles having a particle diameter in the range20 to 300 microns and having a particle diameter size distribution of atleast 20 microns, preferably at least 50 microns and more preferably atleast 100 microns, the fluidised bed may be maintained in a turbulentregime by using solid particles having a particle density of at least0.6 g/cm³ and a settled bed density of at least 0.4 g/cm³ and byoperating at a pressure in the fluid bed reactor of at least 4 bara witha total linear gas flow through the fluidised bed of from 2 to 30 cm/sinclusive, preferably 2-25 cm/s inclusive.

Thus, according to one aspect of the present invention there is provideda process for the production of vinyl acetate by the reaction ofethylene and acetic acid with molecular oxygen in a fluid bed reactor inthe presence of a catalytically active fluidised bed of solid particleshaving a particle diameter in the range 20 to 300 microns and having aparticle diameter size distribution of at least 20 microns, preferablyat least 50 microns and more preferably at least 100 microns, and inwhich process a molecular oxygen-containing gas having an oxygenconcentration greater than that of air is introduced into the fluidisedbed, and the fluidised bed is maintained in a turbulent regime by usingsolid particles having a particle density of at least 0.6 g/cm³ and asettled bed density of at least 0.4 g/cm³ and by operating at a pressurein the fluid bed reactor of at least 4 bara with a total linear gas flowthrough the fluidised bed of from 2 to 30 cm/s inclusive, preferably2-25 cm/s inclusive.

Preferably, in such a process for the production of vinyl acetate, thefluid bed comprises catalyst particles which have a particle sizedistribution in which (i) at least 65% of the particles have particlediameters in the range 20-120 microns, (ii) less than 15% of theparticles have a particle diameter of less than 45 microns and (iii)less than 5% of the particles have a particle diameter of greater than105 microns.

As the linear gas flow rate is increased, so increasingly dense catalystparticles will be required to reduce transportation of catalystparticles from the fluidised bed.

Suitably, the catalyst particles useful for the production of vinylacetate have a particle density in the range 1.1 to 1.5 g/cm³. Thedensity of the particles may be changed by changing the amount of liquidcomponents such as acetic acid introduced into the fluid bed of catalystduring the process.

When the process of the present invention is used for the reaction ofmolecular oxygen with (a) ethane to produce ethylene and/or acetic acid,(b) ethylene to produce acetic acid, or (c) mixtures of ethane andethylene to produce acetic acid, optionally with ethylene, in a fluidbed reactor in the presence of a catalytically active fluidised bed ofsolid particles having a particle diameter in the range 20 to 300microns and having a particle diameter size distribution of at least 20microns, preferably at least 50 microns and more preferably at least 100microns, the fluidised bed may be maintained in a turbulent regime byusing solid particles having a particle density of at least 0.6 g/cm³and a settled bed density of at least 0.4 g/cm³ and by operating at apressure in the fluid bed reactor of at least 4 bara with a total lineargas flow through the fluidised bed of from 2 to 35 cm/s, inclusive.

Thus, according to one aspect of the present invention there is provideda process for the reaction of molecular oxygen with (a) ethane toproduce ethylene and/or acetic acid, (b) ethylene to produce acetic acidor (c) mixtures of ethane and ethylene to produce acetic acid,optionally with ethylene, in a fluid bed reactor in the presence of acatalytically active fluidised bed of solid particles having a particlediameter in the range 20 to 300 microns and having a particle diametersize distribution of at least 20 microns, preferably at least 50 micronsand more preferably at least 100 microns, and in which process amolecular oxygen-containing gas having an oxygen concentration greaterthan that of air is introduced into the fluidised bed, and the fluidisedbed is maintained in a turbulent regime by using solid particles havinga particle density of at least 0.6 g/cm³ and a settled bed density of atleast 0.4 g/cm³ and by operating at a pressure in the fluid bed reactorof at least 4 bara with a total linear gas flow through the fluidisedbed of from 2 to 35 cm/s, inclusive.

Preferably, in such a process for the reaction of ethane and/or ethylenewith molecular oxygen to produce ethylene and/or acetic acid, thefluidised bed comprises particles in which (i) at least 65% of theparticles have particle diameters in the range 20-120 microns, (ii) lessthan 15% of the particles have a particle diameter of less than 45microns and (iii) less than 5% of the particles have a particle diameterof greater than 105 microns.

Suitably, the catalyst particles used for the oxidation of ethane and/orethylene to produce ethylene and/or acetic acid have a particle densityof 1.25 to 3.5 g/cm³, preferably 1.75 to 2.25 g/cm³.

It has been found that such a process according to the present inventionfor the production of vinyl acetate can be readily scaled up becausedata obtained on a small scale is applicable to the full scale becauseboth fluid beds operate in a turbulent regime.

In the present invention it has been found that by operating at highpressures (for example at least 4 bara), a turbulent regime may bemaintained in the fluid bed with a small amount (for example less than15%, preferably less than 5%) of solid particles having a particlediameter of less than 45 microns, or even less than 20 microns (fines).

The use of low levels of fines provides an advantage of reduced loss ofthe particles from the reactor and may thus provide advantages of designof any cyclones to reduce loss of material from the reactor.

In other processes according to the present invention, it may bepossible to maintain a turbulent regime in the fluid bed by using afluid bed comprising solid particles with greater than 15% of theparticles having a particle diameter of less than 45 microns, forexample 30 to 40% inclusive and at the same time operating at a lowerpressure, for example at 0.5 to 1 barg, inclusive and a higherfluidising gas velocity.

The density of the solid particles of the fluid bed may be modified bythe use of a densifier during preparation of the particles.Densification of the particles may be achieved by substitution of one ormore metals of the catalyst by at least one metal which is of higheratomic weight and, preferably, which is also chemically equivalent, thatis, has the same or similar activity and/or selectivity to thesubstituted metal(s). For example, a catalyst comprising the metals, Mo,V, Nb may be densified by substituting one or more of Mo, V, or Nb by atleast one metal of higher atomic weight such as Sn, Sb, Te, La, Ta, W,Re, Pb, Bi, Ce, Pr, Gd and Yb.

Alternatively, the density of the solid particles of the fluid bed maybe modified by the use of a fluidising gas, such as ethylene. Thefluidising gas may comprise at least 30 mol % ethylene by volume, forexample, at least 30 mol % to 85 mol % ethylene.

Increasing the density of the fluid bed solid particles, for example byusing a densifier, may enable higher superficial gas velocities to beused and/or higher levels of fines whilst maintaining the fluid bed in aturbulent, non-transporting regime.

The process of the present invention allows for the safe introductioninto a fluid bed, of a molecular oxygen-containing gas having an oxygenconcentration of up to almost pure molecular oxygen. Suitably, molecularoxygen-containing gases for use in the present invention include oxygengas with minor amounts of impurities such as argon and nitrogen whicheach may be present at a concentration of less than 0.1% by volume. Theconcentration of oxygen in the molecular oxygen-containing gas issuitably in the range greater than 21% to 100%, preferably in the range50 to 100%, for example a concentration of greater than 99.5% by volume,suitably a concentration of at least 99.6% by volume.

The means for introducing the molecular oxygen-containing gas into thefluidised bed should recognise the hazards which may have to beconsidered with such materials. Thus, for example, for safety, theinlet(s) for the molecular oxygen-containing gas are preferably locatedat a distance from the catalyst support means of greater than anypotential flame length. The potential flame length is determined byfactors such as the inlet pipe diameter and inlet gas velocity. Theinlets should be positioned and inlet pressures and velocities selected,so that the molecular oxygen-containing gas is dispersed and mixed inthe region of the inlet. The inlets should be positioned not too closeto the reactor walls, in case there is a shock wave following adetonation. The inlets should be positioned so that the molecularoxygen-containing gas does not impinge directly on surfaces or otherstructures in the reactor such as inlets for other reactants.

In the process of the present invention the at least one oxidisablereactant may be introduced into the fluidised bed as a gas and/or liquidthrough one or more inlets.

One or more gaseous reactants may be introduced into the fluidised bedtogether with or preferably at least in part, separately from, themolecular oxygen-containing gas.

This gaseous reactant may be introduced as a component of the fluidisinggas. The fluidising gas may comprise fresh gaseous reactants and/orrecycle gases. The gaseous reactant introduced into the reactor may beethylene which may be used with acetic acid to produce vinyl acetate.The gaseous reactant introduced into the reactor may be ethane and/orethylene which may be oxidised respectively to ethylene and/or aceticacid and to acetic acid.

Ethylene used in the process of the present invention may be used insubstantially pure form or admixed with one or more of nitrogen,methane, ethane, carbon dioxide and water in the form of steam or one ormore of hydrogen, C₃/C₄ alkenes or alkanes. Ethane used in the processof the present invention may be used in substantially pure form oradmixed with one or more of nitrogen, methane, ethylene, carbon dioxideand water in the form of steam or one or more of hydrogen, C₃/C₄ alkenesor alkanes.

The process of the present invention may suitably be operated at atemperature from 100 to 500° C., preferably 140 to 400° C. The processmay suitably be operated at a pressure of 10 to 3000 kPa gauge (0.1 to30 barg), preferably 20 to 2500 Pa gauge (0.2 to 25 barg).

The fluidised bed may comprise catalyst particles and optionallycatalytically inactive particles. The catalyst may be any suitablefluidisable catalyst. The catalyst may be a supported catalyst. Suitablecatalyst supports include porous silica, alumina, silica/alumina,titania, silica/titania, zirconia and mixtures thereof. Preferably, thesupport is silica.

A typical catalyst composition useful in this invention, may have thefollowing particle size distribution:—

0 to 20 microns 0-30 wt % 20 to 44 microns 0-60 wt % 44 to 88 microns10-80 wt %  88 to 106 microns 0-80 wt % >106 microns 0-40 wt % >300microns  0-5 wt %

Persons skilled in the art will recognise that support particles sizesof 44, 88, 106 and 300 microns are arbitrary measures in that they arebased on standard size ranges such as may be measured by an automatedlaser device such as a Microtrac X100.

Preferably, the fluidised bed has a particle density of at least 0.6g/cm³. Catalyst for the production of vinyl acetate may have a densityin the range 1.1 to 1.5 g/cm³. Catalyst for the oxidation of ethaneand/or ethylene suitably may have a density in the range 1.25 to 3.5g/cm³, preferably in the range of 1.75 to 2.25 g/cm³. Suitably, thefluidised bed has a settled bed density of at least 0.4 g/cm³.

Suitable catalysts for use in the present invention include oxidation,ammoxidation and carboxylation catalysts.

A catalyst suitable for use in the production of vinyl acetate by theacetoxylation of ethylene may comprise a Group VIII metal, a catalystpromoter and an optional co-promoter. The catalyst may be prepared byany suitable method, such as that described in EP-A-0672453. The GroupVIII metal is preferably palladium. The Group VIII metal may be presentin a concentration of greater than 0.2% by weight, preferably greaterthan 0.5% by weight based upon total weight of catalyst. The metalconcentration may be as high as 10% by weight. Suitable promotersinclude gold, copper, cerium or mixtures thereof. A preferred promoteris gold. The promoter metal may be present in an amount of from 0.1 to10% by weight in the finished catalyst. Suitable co-promoters includeGroup I, Group II, lanthanide or transition metals, for example selectedfrom the group consisting of cadmium, barium, potassium, sodium,manganese, antimony, lanthanum and mixtures thereof, which are presentin the finished catalyst as salts, e.g. an acetate salt. The preferredsalts are potassium or sodium acetate. The co-promoter is preferablypresent in the catalyst composition in a concentration of 0.1 to 15% byweight of catalyst, more preferably, from 1 to 5% by weight. When aliquid acetic acid feed is used, the preferred concentration ofco-promoter salt is up to 6% by weight, especially 2.5 to 5.5%. Wherethe acid is introduced in the vapour phase the co-promoter salt ispreferably present in a concentration up to 11 wt %.

A catalyst composition suitable for the selective oxidation of ethaneand/or ethylene to acetic acid is described in EP-A-1069945, thecontents of which are hereby incorporated by reference. This describes acatalyst composition comprising in combination with oxygen theelements:—Mo_(a).W_(b).Ag_(c).Ir_(d).X_(e).Y_(f)  (I)wherein X is the elements Nb and V;Y is one or more elements selected from the group consisting of:

Cr, Mn, Ta, Ti, B, Al, Ga, In, Pt, Zn, Cd, Bi, Ce, Co, Rh, Cu, Au, Fe,Ru, Os, K, Rb, Cs, Mg, Ca, Sr, Ba, Zr, Hf, Ni, P, Pb, Sb, Si, Sn, Ti, U,Re and Pd;

a, b, c, d, e and f represent the gram atom ratios of the elements suchthat

0<a≦1, 0≦b<1 and a+b=1;

0<(c+d)≦0.1;

0<e≦2; and

0≦f≦2.

Another catalyst composition suitable for the oxidation of ethane toethylene and/or acetic acid and/or for the oxidation of ethylene toacetic acid is described in EP-A-1043064, the contents of which arehereby incorporated by reference. This describes a catalyst compositioncomprising in combination with oxygen the elements molybdenum, vanadium,niobium and gold in the absence of palladium according to the empiricalformula:Mo_(a)W_(b)Au_(c)V_(d)Nb_(e)Y_(f)  (I)wherein Y is one or more elements selected from the group consisting of:Cr, Mn, Ta, Ti, B, Al, Ga, In, Pt, Zn, Cd, Bi, Ce, Co, Rh, Ir, Cu, Ag,Fe, Ru, Os, K, Rb, Cs, Mg, Ca, Sr, Ba, Zr, Hf, Ni, P, Pb, Sb, Si, Sn,Tl, U, Re, Te, and La;a, b, c, d, e and f represent the gram atom ratios of the elements suchthat:

0<a≦1; 0≦b<1 and a+b=1;

10⁻⁵<c≦0.02;

0<d≦2;

0<e≦1; and

0≦f≦2.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be illustrated by reference to the followingexamples and FIGS. 1 to 4 in which

FIG. 1 represents in schematic form apparatus for measuring thefluidisation characteristics of a fluid bed reactor by X-ray imaging,

FIG. 2 represents in graph form the effect of the level of fines in thefluid bed on the fluid bed density,

FIG. 3 represents in graph form the effect of fluidising gas velocity onthe fluid bed bulk density and

FIG. 4 represents in graph form the effect of ethylene on the fluid beddensity.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Apparatus X-RayEquipment

A series of experiments were performed to study the fluidisationcharacteristics of a fluid bed using the apparatus shown schematicallyin FIG. 1. The fluid bed was fluidised with inert gas (nitrogen) ratherthan reactant gases to enable the fluidisation characteristic of the bedto be studied. Referring to FIG. 1, the apparatus comprised a 420 mm IDaluminium vessel (7), a gas feed system (8) and recycle gas system (9).The vessel contained a bed (1) of fluidisable solid particles supportedon a distributor plate (2) through which fluidising nitrogen gas waspassed from the gas feed (8) and recycled systems (9) was fluidised withnitrogen from a distributor grid plate (2). The vessel was connected toa positive displacement gas circulation pump (3), heater (4) and gasmetering/measuring system (not shown) in a pressurised closed loop.Pressure tappings (5) were positioned along the vessel wall to allow thebulk density of slices of the bed to be derived from the differentialpressure across portions of the fluid bed. A section at the top of thebed (6) contained cyclones to prevent solid particles escaping thevessel.

An X-ray gun (13) provided x-rays which passed through the vessel toimage intensifier (10), camera (11) and image processing equipment (12).

Supported vinyl acetate catalysts were used in the experiments and wereprepared as follows:

a) Preparation of Catalyst Support

The support used for the catalyst preparation was prepared byspray-drying a mixture of Nalco (Nalco Chemical Company) silica sol 1060and Degussa (Degussa Chemical Company) Aerosil® silica. In the driedsupport, 80% of the silica came from the sol and 20% of the silica camefrom the Aerosil. The spray-dried microspheres were calcined in air at640° C. for 4 hours. This method of support preparation is described inEP-A-0672453.

The particle size distribution of the support which was used for thecatalyst preparation is as follows:

Particle size % >300 microns  2 88-300 microns 30 44-88 microns 38 <44microns 30b) Preparation of Catalyst A

Silica support (54.4 kg) prepared as hereinbefore described wasimpregnated with a solution of Na₂PdCl₄.xH₂O (containing 1 kg palladium)and AuCl₄.xH₂O (containing 400 g gold) in distilled water by theincipient wetness. The resulting mixture was mixed thoroughly, left tostand for 1 hour and dried overnight.

The impregnated material was added slowly to a 5% solution of hydrazinein distilled water, and the mixture allowed to stand overnight withoccasional stirring. Thereafter the mixture was filtered and washed with4×400 L distilled water. The solid was then dried overnight.

The material was impregnated with an aqueous solution of potassiumacetate (2.8 kg) by incipient wetness. The resulting mixture was mixedthoroughly, left to stand 1 hour and dried overnight.

The resulting catalyst comprised 1.6 wt % palladium, 0.6 wt % gold and 5wt % potassium on silica based on ICP analysis.

Several batches of catalyst were made according to this method andcombined. The catalyst was then screened to generate specific sizedistributions as shown in Table 1.

TABLE 1 Sample 1 Sample 2 Sample 3 Sample 4 <45 microns 10% 15% 20% 25%45-111 microns 80% 77% 76% 72% >111 microns 10%  8%  4%  3%c) Preparation of Catalyst B

Prior to use silica support prepared as hereinbefore described wasclassified to reduce the amount of particles of <45 microns to less than15%. The silica support (1124 kg) was impregnated with a solution ofNa₂PdCl₄.xH₂O (containing 11.4 kg palladium) and AuCl₄.xH₂O (containing4.56 g gold) in distilled water by the incipient wetness technique. Theresulting mixture was mixed thoroughly and dried in a ribbon blenderwith a mean wall temperature of 120° C. During the drying, a dry airpurge was passed through the blender over the agitated material toremove the evolved moisture. Following drying the material was cooled toless than 35° C. The impregnated salts were reduced to metallic state byaddition of the solid material to a stirred aqueous solution ofhydrazine (2200 litres, 5% by weight hydrazine). Thereafter, thematerial was then pumped to a Nutsche pressure filter and filtered undernitrogen. The filter cake was washed 3 times with about 1000 litres ofdemineralised water in each wash.

The wet material (filter cake) was blended with solid anhydrouspotassium acetate (60 kg) in a ribbon blender and thereafter dried underagitation by introducing steam into the steam jacket of the blender togive a mean wall temperature of 120° C. The evolved moisture was removedwith a dry air purge through the blender. Drying was stopped when themoisture content of the material was in the range 20 to 25% by weight.The partially dried material was transferred to a fluid bed drieroperated at an air inlet temperature of 150° C. to remove the remainingmoisture and give a free flowing catalyst material.

The resulting catalyst comprised 0.93 wt % palladium, 0.37% gold and2.11 wt % potassium based on ICP analysis.

The particle size distribution of the finished material was as follows:

Particle size % >110 microns 23 45-110 microns 68 <45 microns  9c) Fluidisation Experiments

(i) Effect of Fines on Fluid Bed Density and Effect of Fluidising GasVelocity on the Fluid Bed Bulk Density.

90 kg of catalyst A prepared as hereinbefore described was loaded intothe aluminium vessel and fluidised with nitrogen at 8 barg and 150-157°C. over a range of gas velocities. Steady state experiments were run(with the cyclones returning any catalyst which reaches the top of thevessel to the catalyst bed). Differential pressure readings were takenfrom which average bed densities were calculated. The results are shownin Table 2 and plotted in graph form in FIGS. 2 and 3.

TABLE 2 VARIATION OF BED DENSITY^(a) WITH FINES CONTENT AND VELOCITYVelocity = Velocity = Velocity = 13 cm/s 17 cm/s 21 cm/s 10% <45micron^(b) 459 424 395 15% <45 micron^(b) 409 432 409 20% <45 micron^(b)375 303 239 25% <45 micron^(b) 332 294 245 ^(a)Bed density (in kg/m³) isthe average of two density measurements made in the first 1.0 m of thefluid bed (measured from the grid plate upwards). ^(b)Fines (defined as<45 micron material) content of the catalyst as loaded into the reactor.

The velocity in Table 2 and in FIGS. 2 and 3 is the superficial gasvelocity of the fluidising gas and is defined as gas flow rate(cm³/s)/fluid bed cross-section area (cm²).

During these experiments, the x-ray images showed that the bed was beingmaintained in a turbulent regime. Thus, the experiments show that thereis a range of operating conditions under which a turbulent regime can bemaintained.

Referring to FIG. 2, this shows the bed density as a function of theamount of fines in the fluid bed at three different superficialvelocities. The data show that as the superficial gas velocity isincreased, the bed density decreases. Also, at any given superficial gasvelocity, as the amount of fines increases so the bed density decreases.There is a sharp reduction in bed density at the higher fines content(>20%) and high gas velocity (>21 cm³/s) indicating that the bed isentering the transport regime, where it occupies all the available spacein the vessel.

Referring to FIG. 3, this shows the bed density as a function of thesuperficial gas velocity for the different amount of fines in the fluidbed. The data show that as the superficial gas velocity is increased,the bed density decreases. There is a reduction in bed density as thefines content increases, particularly at high gas velocity (>21 cm³/s).This indicates that the level of fines should be kept at no greater than15% to maintain a reasonable bed density and prevent transportation atthe higher gas velocity.

(ii) Effect of Ethylene on the Fluid Bed Density

5.1 kg of catalyst B prepared as hereinbefore described was loaded intoa 2.1 metre tall vessel with a diameter of 3.5 inches. At the top of thevessel a cone shaped expansion section was fitted followed by a straightsided freeboard section with a diameter of 6 inches. The vessel wasequipped with a differential pressure transmitter covering the lower1.38 metres of height. Thus based on the known volume of the lowersection of the vessel and the mass of catalyst in that section from thedifferential pressure transmitter reading the density of the fluidisedbed could be calculated. Two experiments were carried out. The firstexperiment used nitrogen as fluidising gas. The second experiment usedethylene with 10-20 vol % nitrogen as fluidising gas. The experimentswere carried out at 154° C. and at a pressure range of 7.2 to 9.2 barg.

Referring to FIG. 4, this shows the bed density as a function of thesuperficial gas velocity for the different fluidizing gases. The datashows a higher bed density is achieved when ethylene is used as afluidising gas compared to the bed density achieved when nitrogen isused as the fluidising gas. The data also indicates that highersuperficial gas velocities may be achieved when using ethylene as thefluidising gas whilst still in the turbulent regime. It is believed thatthe increased noise in the nitrogen data above 19.5 c m/sec is due tothe bed transporting.

Further experiments were performed to investigate the effect of beddensity on the fluidisation characteristics.

Effect of Bed Density on Fluidisation.

Catalysts containing a densifier were prepared and tested as follows.

a) Preparation of Support

EXAMPLES Example 1 (Comparative)

The support was prepared by spray-drying a mixture of Nalco (NalcoChemical Company) silica sol 1060 and Degussa (Degussa Chemical Company)Aerosil® silica. In the dried support, 80% of the silica came from thesol and 20% of the silica came from the Aerosil. The spray-driedmicrospheres were calcined in air at 640° C. for 4 hours. This method ofsupport preparation is described in EP-A-0672453.

Example 2

The support was prepared as in Example 1 except that 90% of the silicacame from the sol and 10% came from the Aerosil

Example 3

The support was prepared as in Example 2 except that the Aerosil wasreplaced by tin oxide.

The particle size distributions of the supports are as follows:

Particle size % Example 1 % Example 2 % Example 3 >350 microns 0.8 0.3 088-350 microns 27.4 15.1 12.5 40-88 microns 33.8 31.5 31.5 <40 microns38.0 53.1 56.0b) Preparation of Catalyst

30.0 g of microspheroidal support was impregnated with a solution ofNa₂PdCl₄.xH₂O (1.84 g, 28.9% Pd) and HAuCl₄.xH₂O (0.44, 49.0% Au) indistilled water (20.0 cm³) by incipient wetness. The resulting mixturewas mixed thoroughly, left to stand for 1 hour and then dried overnight.

The impregnated material was allowed to cool to room temperature, andthen was added to a solution of hydrazine (3.0 g of a 55% aqueoussolution of N₂H₄) in 80.0 cm³ distilled water, and the mixture allowedto stand overnight with occasional stirring. Thereafter the mixture wasfiltered and washed with 4×250 cm³ distilled water. The solid was thendried overnight.

The material was impregnated with an aqueous solution of potassium (1.64g) by incipient wetness. The resulting mixture was mixed thoroughly,left to stand for 1 hour and dried overnight.

The catalyst of Example 1 had a metal loading of 1.49% Pd, 0.44% Au and3.64% potassium acetate. The catalyst of Example 2 had a metal loadingof 1.65% Pd, 0.53% Au and 4.54% potassium acetate. The catalyst ofExample 3 had a metal loading of 1.63% Pd, 0.54% Au and 4.49% potassiumacetate, 0.47% Sn.

c) Production of Vinyl Acetate

The catalysts were tested in a bench scale fluid bed reactor with amaximum catalyst capacity of 40 cm³. For each experiment 30 cm³ ofcatalyst-plus-diluent was loaded into the reactor. Typically 2 g ofcatalyst was used. The inert diluent consisted of silica loaded withgold and potassium acetate. The reactor was equipped with two feedinlets. Ethylene, acetic acid, and oxygen all entered the reactorthrough a lower inlet and nitrogen only was fed through the second inletwhich was located 2.5 inches above the first.

The reactor pressure was controlled at 8 barg and all lines leading toand from the reactor were heat traced and maintained at 150-160° C. inorder to prevent condensation of liquid feeds or products. The reactorwas normally maintained at a temperature of 150-160° C.

The gaseous reactor effluent was analysed on-line using a HewlettPackard Model 5890 gas chromatograph equipped with both TCD and FIDdetectors. Oxygen, nitrogen, ethylene and carbon dioxide were separatedon a 13× mole sieve column parallel with 10% Carbowax 20M on 80/100Chromosorb WAW and 23% SP2700 on 80/100 Chromosorb PAW, and quantifiedwith the TCD. Vinyl acetate and acetic acid and other organicby-products were separated on a J&W DB 1701 megabore capillary columnand quantified with the FID. Data was analysed via a customised Excelspreadsheet.

Settled Bulk Density Catalyst Activity Example of Support g/cm³gVA/kg-cat/hr Selectivity Example 1 run 1 0.66 1420 91.8 Example 1 run 20.66 1270 93.1 Example 2 0.80 1280 90.6 Example 3 run 1 0.94 1270 89.4Example 3 run 2 0.94 1200 92.8

The data show that catalysts with higher densities can be made by addinginert oxides during support preparation, and such catalysts have similarintrinsic activity to catalysts made without a densifier. Thus, in viewof the fluidisation experiments, it can be concluded that increasing thedensity of the fluid bed solid particles, for example by using adensifier, may enable higher superficial gas velocities to be usedand/or higher levels of fines whilst maintaining the fluid bed in aturbulent, non-transporting regime.

1. A process for reacting in a fluid bed reactor, at least oneoxidisable hydrocarbon reactant which is not ethane or ethylene orpropylene with molecular oxygen in the presence of a catalyticallyactive fluidised bed of solid particles, which process comprisesintroducing a molecular oxygen-containing gas having an oxygenconcentration greater than that of air into said fluidised bed whilstsaid fluidised bed is maintained in a turbulent regime, wherein theturbulent regime has the ratio of transition velocity (U_(k)):terminalvelocity (U_(t)) in the range 0.1:1 to 25:1.
 2. A process according toclaim 1 wherein the hydrocarbon is a C₄ hydrocarbon and the C₄hydrocarbon is reacted with molecular oxygen to produce maleicanhydride.
 3. A process according to claim 1 wherein the hydrocarbon isreacted with molecular oxygen and ammonia.
 4. A process according toclaim 3 wherein the hydrocarbon is propane, and the hydrocarbon isreacted with molecular oxygen and ammonia to produce acrylonitrile.
 5. Aprocess according to claim 1 wherein the solid particles have a particlediameter in the range 20 to 300 microns.
 6. A process according to claim3 wherein the particle diameter size distribution is at least 20microns.
 7. A process according to claim 1 wherein the fluidised bedcomprises solid particles having sizes according to one or more of thefollowing independent criteria: (i) at least 65% of the particles haveparticle diameters in the range 20-120 microns; (ii) less than 15% ofthe particles have a particle diameter less than 45 microns; and (iii)less than 5% of the particles have a particle diameter greater than 105microns.
 8. A process according to claim 1 wherein the concentration ofoxygen in the molecular oxygen-containing gas is in the range 50 to 100%by volume.
 9. A process according to claim 1 wherein the at least oneoxidisable reactant is introduced into the fluidised bed as a gas and/orliquid through one or more inlets.
 10. A process according to claim 9wherein the at least one oxidisable reactant is introduced into thefluidised bed as a gas and is a component of the fluidising gas.
 11. Aprocess according to claim 1 wherein the fluidised bed has a particledensity of at least 0.6 g/cm³.
 12. A process according to claim 1wherein the fluidised bed has a settled bed density of at least 0.4g/cm³.
 13. A process according to claim 1 wherein the solid particlescomprise a support.
 14. A process according to claim 13 wherein thesupport is selected from the group consisting of silica, alumina,silica/alumina, titania, silica/titania, zirconia and mixtures thereof.15. A process according to claim 1 wherein the particle density of thesolid particles is modified by the use of a densifier during thepreparation of the particles.
 16. A process according to claim 15wherein the densifier is used during preparation of the support.
 17. Aprocess according to claim 16 wherein the densifier is an inert oxide.18. A process according to claim 17 wherein the inert oxide is tinoxide.
 19. A process according to claim 16 wherein the particle densityis modified by substitution of at least one metal of the solid particlesby at least one metal having a higher atomic weight.
 20. A processaccording to claim 19 wherein the at least one substituting metal ischemically equivalent to the at least one substituted metal.
 21. Aprocess according to claim 20 wherein the solid particles comprise atleast one of Mo, V and Nb, and the particle density of the particles ismodified by substituting at least one of Mo, V, Nb by at least one ofSn, Te, La, Ta, W, Re, Pb, Bi, Ce, Pr, Gd and Yb.
 22. A processaccording to claim 1 wherein the particle density of the solid particlesis modified by the use of a fluidising gas.